The present invention relates to the making of iron powder and more particularly the invention relates to a method and equipment for atomizing molten iron by means of a water jet ejected from an anular nozzle. The invention relates further to powder made by means of such a method and such equipment and being characterized by a high degree of compressibility or compactability and low bulk density to be particularly usable for powder press working.
Generally speaking iron powder which is used in powder metallurgy and worked by means of powder presses can be classified as follows:
(a) light powder having a bulk or apparent density of about 2.5 grams per cubic centimeter and being normally compressible for compacting in that, for example, six metric tons per square centimeter increases the density to 6.8 grams per cubic centimeter;
(b) heavy powder having a bulk or apparent density in excess of 3.0 grams per cubic centimeter and a high compressibility in that, for example, six metric tons per square centimeter increases the density to 7.1 grams per cubic centimeter.
Powder as per class (a) above results from directly reducing iron ore or by atomizing highly carbonized molten iron by means of pressurized air. Upon directly reducing iron ore rather spongy materials are produced. Air atomization followed by annealing results in more or less hollow spherical powder particles.
Power as per class (b) is produced by atomizing low carburized steel by means of pressurized water. The powder particles are quite compact by themselves and they are more or less spattered depending, for example, on the water pressure. This method is described, for example, in U.S. Pat. Nos. 2,892,215 and 3,325,277.
As stated, light weight powder is characterized by medium compressibility and by a high green strength of the products resulting from compacting the powder by and in a press into particularly shaped articles. The heavy powder grades exhibit a higher compressibility, but the resulting products have an unsatisfactory green strength, particularly for lower compacting densities such as below 6.5 grams per cubic centimeter.